As a vinyl alcohol-based polymer (hereinafter, may be abbreviated as a PVA) having an ethylenic double bond introduced into the molecule, those described in Patent Documents 1 to 5 below are known. Patent Document 1 describes a PVA post-modified with a reactive molecule containing an ethylenic double bond, thereby having an ethylenic double bond introduced into the side chain of the PVA. Patent Document 2 describes a product obtained by, after obtaining a vinyl ester-based copolymer having a protected ethylenic double bond, removing the protection and saponifying it. Patent Document 3 describes a PVA having an ethylenic double bond introduced into an end of the molecular chain using aldehyde or the like as a chain transfer agent. Patent Document 4 describes a product obtained by washing and drying after obtaining a carboxyl group containing PVA by saponification after copolymerizing a maleic acid derivative and a vinyl ester monomer, thereby randomly introducing an ethylenic double bond using the carboxyl group as a starting point into the main chain. Patent Document 5 describes a product obtained by washing and drying what is saponified after copolymerizing carbon monoxide and a vinyl ester monomer, thereby introducing an enone structure into the main chain.
These PVAs having an ethylenic double bond introduced into the molecule are widely utilized as a photosensitive material and a dispersion stabilizer for suspension polymerization and emulsion polymerization of a vinyl-based monomer. Among all, when these PVAs are used as a dispersion stabilizer for polymerization, the ethylenic double bond present in the molecular chain of the PVA is involved in chemical bond formation with dispersoids and high polymerization stability is exhibited even in a small amount of use.
However, in the production methods described in Patent Documents 1 and 2 above, a step for introducing an ethylenic double bond into the PVA is separately necessary other than the step of producing a PVA. Performance of them is therefore limited from the economic perspective. When an ethylenic double bond is introduced into the main chain of the PVA molecule by the production methods described in Patent Documents 3, 4, and 5 above, such PVAs are easily colored by heating and thus their use is limited. In addition, when such PVAs are used as dispersion stabilizers for polymerization, there is a problem of coloration of the polymers to be produced.